Matching circuits are widely used to transform the impedance of the various components within a circuit to a target impedance. The input to the matching circuit may be an RF signal, carrying information. The output to the matching circuit may also be an RF signal, carrying information. The matching circuit may precede or follow a component. The component has an input and an output impedance. If the component follows the matching circuit in the signal path, the target impedance of the component is its input impedance. If the component precedes the matching circuit in the signal path, the target impedance of the component is its output impedance.
The matching circuit functions to set the impedance seen by the signal to the target impedance by compensating for the difference between the impedance of interest and the target impedance. The compensation of the impedance is determined by the capacitance and the inductance of the matching circuit and by the configuration of the matching circuit. A wide range of impedance matching and transfer function circuits can be realized by using lumped element inductors or capacitors or both. At higher frequency (above about 1 GHz) it is often advantageous to replace either lumped element inductors or capacitors or both with distributed transmission line networks. The usefulness of this replacement is also dependent on the dielectric constant (DK) of the substrate, as well as area constraints.
Different arrangements of matching circuits are well known in the art. Some examples are: series capacitor, shunt capacitor; series capacitor, shunt inductor; series inductor, shunt capacitor; etc. A common configuration for a matching circuit is shunt capacitor, series inductor, shunt capacitor. Tranformers, and even resistive networks, can be used, if the insertion loss can be tolerated.
In general, matching circuits should have minimum added loss to prevent added degradation in the information signal. Excess loss increases the demands made on other components in an electronic system, especially the active elements such as amplifiers. In some cases, like at the input to a low noise amplifier (LNA), increased signal loss cannot be made up (compensated for) by simply increasing the gain of the LNA.
The impedances of the components and the matching circuits are frequency dependent. The impedance is only matched at a single operating frequency, or over a limited band of frequencies. If the designer wishes to operate the device at more than one frequency band, compromises must be made.
Antenna matching networks in wireless handsets will serve as an example. Since present dual band handsets typically use only a single antenna, the present solution is to match impedances at a frequency in the middle of the range of operating frequencies. The impedance is mismatched at all frequencies besides the middle frequency. This increases power loss in either band.
A second option is to use different components and matching circuits for different operating frequencies. The impedance match may be better this way, but the extra cost of manufacturing and overall size of the combined circuit may be prohibitive. It will be appreciated that if one of each component could be well matched at all desired operating frequencies, there would be a great savings in circuit size and power consumption. A tunable matching circuit would solve this problem.
In code division multiple access (CDMA) handsets, matching circuits are required between the duplexer and antenna (single band phone) or the diplexer or multiplexer and antenna in dual or multi-band handsets as well as before and after power amplifiers (PA""s) and low noise amplifiers (LNA""s).
Prior to the invention, low loss tunable matching circuits at frequencies above about 200 MHz had not been achieved, although attempts had been made. Those attempts include matching circuits using varactor diodes as shown in U.S. Pat. No. 6,198,441 B1, hereby incorporated by reference. Specifically, a circuit comprising a series inductor and a shunt varactor diode has been used to make tunable matching circuits. The problem with this circuit though, is that the losses are intolerably high, particularly for use in a portable communication device. Further, varactor diodes are quite temperature sensitve, and have proven to possess uncertain rf performance from lot to lot. It will be appreciated that a low loss tunable matching circuit would be useful in many applications, but particularly in a portable wireless communication device.
The objective of this invention is to provide a low loss tunable matching circuit. The matching circuit may be for use in portable wireless communication devices. Low-loss tunable f-e components may be used to make a matching circuit. This may be accomplished by using a f-e tunable capacitor or inductor. Matching circuits can be implemented by lumped elements placed in series or shunt or by distributed network elements or by some combination of the two. In distributed element matching, f-e films can be used in planar (microstrip, stripline, CPW, among others) passive matching circuits to vary the permittivity of the underlying substrate, thus effecting a change in the matching circuit""s or resonator""s electrical length. The use of planar matching circuits is familiar to anyone trained in the art of amplifier or circuit design. The matching networks here can be hybrids and couplers as well as the more conventional distributed inductive and capacitive structures.
The advantages of the invention include lower insertion loss of the matching circuit and a better impedance match resulting in lower insertion loss of the component or components being matched. Furthermore, better matching means that there will be less interference due to reflection within a circuit.
Additionally, portable wireless communication devices will have longer battery lifetimes and talk times.